docs(select): remove solved known issue

The issue in question has been resolved some time in 2017.
The bug report is still open, but the behavior has changed:
https://bugzilla.mozilla.org/show_bug.cgi?id=126379

Let's hope they have tests for this!

Author: Narretz

src/ng/directive/select.js

@@ -556,18 +556,6 @@ var SelectController =
  * {@link guide/interpolation#-ngattr-for-binding-to-arbitrary-attributes ngAttr} directive.
  *
  *
- * @knownIssue
- *
- * In Firefox, the select model is only updated when the select element is blurred. For example,
- * when switching between options with the keyboard, the select model is only set to the
- * currently selected option when the select is blurred, e.g via tab key or clicking the mouse
- * outside the select.
- *
- * This is due to an ambiguity in the select element specification. See the
- * [issue on the Firefox bug tracker](https://bugzilla.mozilla.org/show_bug.cgi?id=126379)
- * for more information, and this
- * [Github comment for a workaround](https://github.com/angular/angular.js/issues/9134#issuecomment-130800488)
- *
  * @example
  * ### Simple `select` elements with static options
  *